Glass is commonly manufactured by melting batch materials and crushed cullet at very high temperatures (upwards of 1500.degree. C.) using electric-assisted natural gas- or fuel oil-fired furnaces. Traditionally, air was widely used as the oxidant in such furnaces, because of its ready availability and low cost. A significant disadvantage associated with the use of air in industrial furnaces is the high concentration of nitrogen oxides (NO.sub.x) that are produced in the furnaces and released into the atmosphere.
In recent years, with the passage of stringent environmental laws and regulations, industry has been required to find alternatives to processes that result in the release of large quantities of gaseous pollutants to the atmosphere. In the glassmaking industry the amount of NO.sub.x released to the environment can be considerably lessened by the use of oxygen or oxygen-enriched air as the oxidant in glassmaking furnaces. Among the many U.S. patents that discuss the use of oxygen or oxygen-enriched air as the oxidant in glassmaking furnaces are U.S. Pat. Nos. 3,337,324, 3,592,622, 3,592,623 and 3,627,504.
A potential advantage of using oxygen or oxygen-enriched air in glassmaking furnaces is the opportunity to produce high purity carbon dioxide from the furnace exhaust gas. When oxygen-enriched air or substantially pure oxygen is used as the oxidant, the exhaust gas usually contains about 30 to 50% carbon dioxide, about 40 to 60% water vapor, and only 0 to about 3% each of oxygen, argon and nitrogen. Thus, the gas is a good source of carbon dioxide. However, the exhaust gas also contains about 500 to 3500 ppm NO.sub.x and about 500 to 1000 ppm sulfur oxides (SO.sub.x). These impurities, together with residual particulate impurities, such as sulfur salts, must be substantially completely eliminated from the gas in order for the gas to meet the standards set for high purity carbon dioxide. For example, food grade carbon dioxide should not contain more than 5 ppm NO.sub.x or more than 1 ppm sulfur compounds. Unfortunately, there are currently no commercial scale cost-effective methods of reducing NO.sub.x and SO.sub.x in gas streams to these levels.
U.S. Pat. No. 4,806,320, discloses the reduction of NO.sub.x in flue gases by mixing ammonia or methane with the flue gas and passing it through a bed of vermiculite. This patent discusses the disclosures of several prior art patents, such as U.S. Pat. Nos. 3,118,727 and 3,806,582, which teach mixing methane with waste gases to reduce the concentration of NO.sub.x in the waste gases; U.S. Pat. Nos. 3,864,451 and 3,008,796, which teach the reduction of NO.sub.x in flue gases by mixing ammonia with the flue gas and contacting a catalyst with the mixture; and U.S. Pat. No. 3,880,618, which teaches the removal of both NO.sub.x and SO.sub.x from flue gas by passing the gas over an alkali metal carbonate, such as sodium carbonate. The above processes are somewhat successful for the removal of NO.sub.x and SO.sub.x from flue gases, but are not always satisfactory for reducing NO.sub.x concentration in flue gases to the levels required under the current stringent environmental regulations and to meet standards set for food-grade carbon dioxide purity.
U.S. Pat. No. 5,149,512 discusses the reduction of NO.sub.x in flue gases by a technique known as Selective Catalyst Reduction (SCR), which involves mixing ammonia with a flue gas and passing the mixture over a catalyst. Also disclosed in this patent is a modified SCR process in which a mixture of a hydrocarbon, such as methane, and flue gas is passed over aluminum-supported platinum, palladium or rhodium catalysts.
Although the above-discussed patents disclose processes for reducing one or more pollutants from flue gases, none of these references disclose processes which efficiently and inexpensively reduce all of the impurities contained in glassmaking furnace flue gas to levels set by the Environmental Protection Agency, and at the same time produce food grade carbon dioxide. The present invention provides a method of more efficiently removing NO.sub.x, SO.sub.x and particulates from oxyfuel-fired glassmaking furnaces than is done in prior art processes, while at the same time producing carbon dioxide which meets food grade standards.